Ultrasonic flow measuring devices are applied often in process and automation technology. They enable contactless determination of the volume, and/or mass, flow rate of a medium in a containment, especially in a pipe.
Known ultrasonic measuring devices work either by the Doppler principle or the travel-time-difference principle. In the case of the travel-time-difference principle, the different travel time of the ultrasonic measuring signals in the direction of medium flow, and counter to the direction of medium flow, is exploited. To this end, the ultrasonic measuring signals are alternatingly issued, and received, in the direction of flow, and counter to the direction of flow. On the basis of the travel-time-difference of the ultrasonic measuring signals, the flow velocity can be determined, and, with that and known diameter of the pipe, the volume flow rate of the medium, or, with known density, the mass flow rate of the medium.
In the case of the Doppler principle, ultrasonic measuring signals of known frequency are coupled into the flowing medium. The ultrasonic measuring signals reflected in the medium are evaluated.
On the basis of a frequency shift occurring between the ultrasonic measuring signal which was coupled into the medium and the reflected ultrasonic measuring signal, likewise the flow velocity of the medium, or the volume, and/or mass, flow rate, can be determined. The use of flow measuring devices working according to the Doppler principle is only possible, when present in the medium are air bubbles or impurities, on which the ultrasonic measuring signals are reflected. Thus, the application of ultrasonic flow measuring devices using the Doppler principle is rather limited, compared to ultrasonic flow measuring devices using the travel-time-difference principle.
With respect to types of measuring devices, a distinction is drawn between ultrasonic flow measuring pickups that are inserted into the pipeline, and those known as clamp-on flow measuring devices, where the ultrasonic transducers are pressed onto the pipeline externally by means of a clamp. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
In both types of ultrasonic flow measuring devices, the ultrasonic measuring signals are radiated at a predetermined angle into, and/or received from, the pipe containing the flowing medium. In order to be able to radiate the ultrasonic measuring signals at a determined angle into, and out of, the pipe, or into, and out of, the medium, the in- and out-coupling of the ultrasonic measuring signals occurs in clamp-on flow measuring devices via an interface piece, or coupling wedge. For achieving an optimum impedance matching, it is, moreover, known to make the coupling wedges of a suitably refracting material, e.g. a synthetic material, or plastic. Principal component of an ultrasonic transducer is usually at least one piezoelectric element, which produces the ultrasonic measuring signals and/or receives them.
It is clear that the ultrasonic measuring signals are received strongly attenuated after passing through the medium and, in the case of clamp-on measuring devices, also through the pipe wall and, as the case may be, due to unfavorable impedance ratios in the in-coupling and out-coupling, respectively, into and out of the medium. In order to obtain usable measurement results, the received ultrasonic measuring signals must, therefore, be suitably amplified. The amplification lies usually in a range of 20-120 dB. The frequency of ultrasonic measuring signals lies in the range of about 100 kHz to 10 MHz. Electronic components working in this frequency range have a relatively high electrical current consumption, i.e. take a relatively high power.